The present invention relates generally to digital data processing and transmission techniques, and particularly to methods for transmitting digital video images.
Systems for transmitting video images by means of electrical or electromagnetic signals are known, and discussed in U.S. Pat. Nos. 5,128,776 and 5,426,513 that are assigned to the assignee hereof and incorporated by reference.
A video image to be transmitted is typically divided into an array of picture elements or pixels. Each pixel represents the video image at one small point of the pixel array. In some systems, a pixel may be represented by a single digital bit, either a zero or a one, indicating either the presence or absence of white in the portion of the image represented by the pixel. In more sophisticated systems, each pixel is represented by plural digital bits which permit each pixel to have more than binary values of zero and one. For example, if four bits are used to represent a pixel then the pixel may have up to sixteen different values, generally ranging from white to black. Finally, in color systems, each pixel may be represented by three sets of plural digital bits, each of the plural digital bits of a set representing the amount of one of the primary colors (red, blue or green) present at the portion of the visual image represented by the pixel. Such a scheme is often utilized in digitizing television images.
A standard broadcast color television picture may be adequately digitized into an image 768 pixels wide and 488 pixels high with each pixel having a depth of 24 bits (8 bits each of red, blue and green). Each screen image then contains approximately 375,000 pixels or approximately 9 million bits of digital data. While such large amounts of data can readily be sent by large bandwidth transmission and receiving equipment, it is often desired to send such digital video images by less expensive and more readily available low speed equipment such as HF (high frequency) radio and telephone voice lines. Such low speed devices typically operate at anywhere from 1200 to 9600 bits per second. If a single digital television image is sent via such a low speed transmission link, for example, a 2400 bps link, approximately 65 minutes would be needed to send a single image. In a tactical military situation, a transmission of such duration is usually desired to be avoided as such a long duration transmission could readily be detected and jammed.
While visual images in the form of television signals may be transmitted quite accurately by VHF equipment, VHF transmissions are generally limited to the line of sight between the transmitter and the receiver. Consequently, it is often desired to send television and other video image signals at other than VHF frequencies. For example, the use of HF signal carriers often permits worldwide communication between transmitter and receiver as the HF signal can be bounced or reflected off the ionosphere and back to earth. However, conventional HF systems do not have sufficient bandwidth to send real time television signals.
Many of the known video image transmission systems transmit an entire screen of data. Often, however, only certain portions of the screen are of interest and some portions of the image may be more important than others. One known system transmits an image and increases image detail as a user zooms in on an area of interest, although the system is not practical for relatively slow data rate transmission systems. See, for example, U.S. Pat. No. 5,321,520 to Inga, et al.
Moreover, selective enhancement techniques of the prior art are relatively fixed in that the operators have little opportunity to determine how the image is to be enhanced after having selected the portions that are to be enhanced. It would, for example, be desirable to afford users with the opportunity to select an enhancement level that may be economically achieved. In some systems, important portions are retransmitted in their entirety, thereby replacing the previous, more compressed, data. To avoid transmission of a portion in its entirety, it is also known to remove some high frequency components, or to select certain, less important, portions of the image for more compression than other portions. However, these systems do not evaluate the differences between the original and the compressed images so that image enhancements may be economically provided. See, for example, U.S. Pat. No. 5,333,212 to Ligtenberg and U.S. Pat. No. 5,289,548 to Wilson, et al.
Accordingly, it is an object of the present invention to provide a novel method of enhancing a compressed digital image that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method of enhancing a digital image that was transmitted with a reduced amount of digital data in which differences between pixels in the uncompressed and the compressed digital image are compared and a statistical distribution of the differences is evaluated to determine which of the differences may be corrected to enhance the digital image more efficiently.
It is yet another object of the present invention to provide a novel method for enhancing a received digital image in which a receiver identifies portions of the digital image that are to be enhanced, in which a transmitter forms an error image for the identified portions, and in which the amount of digital data in the error image is reduced by selecting for transmission parts of the error image in which the amount of digital data that is needed to enhance the received digital image is less than a predetermined amount.
It is still another object of the present invention to provide a novel method of transmitting a digital image that is selectively enhanced in which amount of digital data to be transmitted to enhance the image is selected for transmission based on the pixel-by-pixel differences between pixels in selected portions of the received image and the corresponding pixels in the uncompressed image, and in which at least a predetermined percentage of the pixels are corrected.
It is a further object of the present invention to provide a novel method of enhancing a digital image that was transmitted with a reduced amount of digital data in which differences between pixels in the uncompressed and the compressed digital image may be reduced to zero in selected portions of the image.